Circuit for stabilizing focus of magnetically focused cathoderay tubes



Jan. 18, 1949. J ULMAN 2,459,602

CIRCUIT FOR STABILIZING FOCUS OF MAGNETICALLY FOCUSED CATHODE-RAY TUBESFiled D80. 24, 1946 Regulated B+ INVENTOR Lynn J. Ulmqn.

A'TTORN W Patented Jan. 18, 1949 UNITED STATES CIRCUIT FOR STABILIZKNGFOCUS OF MAGNETICALLY FOCUSED CATHODE- RAY TUBES Lynn J. Ulman,Baltimore, Md, assignor to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvania Application December 24,1946, Serial No. 718,196

3 Claims.

raybeam. This is usually done by means of a focusing coil traversed bydirect current which is substantially coaxial with the major axis of thecathode ray tube. The drawing attached hereto illustrates the generalform of such an arrangement.

With a given voltage impressed on the accelerating anode of the cathoderay tube to impart a definite velocity to the electrons impinging on thescreen, a definite value of current must flow through the focusing coilin order to maintain a sharp focus of the cathode ray beam on thefluorescent screen. In practical cases, both the value of the voltage onthe accelerating anode and the value of current through the focusingcoil are likely to vary in undesired fashion be cause of randomvariations in the voltage of the sources supplying direct current, tochanges in resistance with temperature of the focusing coil, andrtosimilar causes. If the current through the focusing coil is adjusted tothe necessary value to focus the cathode ray beam accurately at thebeginning of operations, the causes above-mentioned are likely to causedefocusing of the beam from time to time during subsequent operation. Itis always expensive, and in many cases impracticable, to provide voltageregulators to maintain the direct current voltage sources so free ofrandom and undesired variations as to maintain the desired degree offocus of the cathode ray beam at all times.

One of the objects of my invention is to provide a novel circuit inwhich the effects of voltage variation on the above-mentioneddirect-current sources on defocusing of the cathode ray beam aresubstantially eliminated.

Another object of my invention is to provide a cathode ray tube andcircuit therefor of such type as to maintain substantially constantfocus in the cathode ray beam at all times during use.

' Still another object of my invention is to provide an arrangementeconomical in first cost and simply adjustable to maintain constant thesize of the trace of the cathode ray beam on the screen of a fluorescentcathode ray tube.

Other objects of my invention will become apparent upon reading thefollowing description, taken in connection with the drawing in which thesingle figure is a schematic diagram of "a circuit supplyingaccelerating voltage and focusing coil currents to a cathode ray tube,in accordance with the principles of my invention.

Referring in detail to the drawing, a cathode ray tube l, which may beof any desired conventional type, has at one end a fluorescent screen 2on which impinges an electron beam produced by acceleration of electronsfrom an electron emissive cathode 3 by an accelerating anode 4. Sucharrangements are conventional in the art and require no extendeddescription. In order to focus the trace of the electron beam toan areaof small cross-section on the screen 2, I provide a focusing coil '5which may also be of a type well known in the art. The voltage for theaccelerating anode 4 is furnished by the output of a full-wave rectifier6 of a type convention-al in the art which may be provided by anysuitable filtering arrangements, this rectifier being supplied withalternating voltage from a conventional alternating-current supplytransformer primary winding 1 connected to an ordinaryalternating-current power source (not shown).

Direct current for the focusing coil 5 is supplied from a secondrectifier 8 energized from the same source, which likewise may besuitably filtered by arrangements conventional in the art. The currentflowing through the focusing coil 5 from the rectifier 8 flows seriallythrough an electron tube 3 of a type conventional in the art, which isprovided with a control electrode I i and a screen grid 12, alsoconventional in the art. The amount of current through the focusing coilmay be initially adjusted by Varying the potential impressed on thecontrol grid I l, which is derived from a suitable potentiometersupplied with direct current from the positive terminal l3 of aregulated voltage supply of any type well known in the art to besuitable.

It can be shown readily that the maintenance of a constant size of thefocal spot of the electron beam on the screen 2 requires that thefollowing equation shall be fulfilled:

H=magnetic field strength in the tube due to the focus coil.

V=-accelerating potential of the cathode ray tube.

K=a constant.

Suppose now that the-electron beam has been adjusted to the desiredfocus initially with a voltage V0 impressed from the voltage source 6and with a magnetic field strength of H0. Now suppose that thealternating-current supply connected to winding 1 undergoes an increaseso that the voltage impressed by the source 6 on accelerating anode 4increases by a small fraction a.

3 Then thefollowingquantity, corresponding to the; left-hand side ofEquation 1, will represent the condition in respect to the focus of thecathode ray beam on the screen 2:

Since a is small, we may neglect a and higher powers in the followingdiscussion.

Since the same source which furnishesvoltage to rectifier 6 likewisefurnishes Voltage to rectifier 8, it is evident that the voltageimpressed by rectifier 8 on the circuit through focusing coil willlikewise increase; and since the circuit through that focusing coilembodies a non-linear impedance (i. e: electron tube 9.), the currentthrough the focusing coil 5: will increase by a small fraction. 1) whichwill, in general, be different fromv a. Hence the conditions at thefocus of; the. cathode ray beam in tube I will be represented by a:modification of the left-hand member of. Equation 1 which will take theform Since: the'f-raction b is a small one, 19 may beneglectedfromfurther consideration.

Itwill; be evident that Equation 1- may then be rewritten; in l the formIt isevident that if we can make: 2b equal to a, the conditions at thecathode ray beam focus, being represented by Ho Vo, will be independentof both b and a. In other words, the focus of the cathode ray beam willbe maintained constant, regardless. of. any small fluctuations which maytake place in the voltage supplied from the power source.

Since the electron tube 9 is a non-linear resistor, it is readilypossible, by. properly adjusting the value of the voltage impressed onthe screen. electrode E2, to insure that the percentage changein currentflowing through the focusing coil 5 is exactly one-halfof the voltagechange impressed by the voltage source 8.: to cause that current flow.The voltage impressed on the screen. electrode I2 is determined bytherelative magnitudes of the resistors M and I5, and by properly vary--ing: these resistors, the condition that the rate of current change inthe plate circuit of tube 9 shall be one-half as. great as thedirect-current voltage supplied to that circuit by source 8 may bereadily fulfilled.

The initial. focusing of the cathode ray beam. on the screen 2 may beeffected by varying the potential impressed by the potentiometer It onthe control electrode II. The potentiometer i6 is supplied with voltagefrom a regulated direct-- current source, which may be of small costbecause it merely supplies voltage to a control electrode.

The time constants of the circuit supplyingvoltage to the screen gridand the circuit supplying accelerating voltage to the anode 4 should beas nearly as possible the same in order that voltage fluctuations shallbe quickly compensated for.

I claim as my invention:

1. In combination with means for producing an 4 electron beam bymeansmfan accelerating anode, a focusingmagnet fOr said electronbeam, a powersupply and means for impressing voltage therefrom on said acceleratinganode, means for impressing voltage. on. said focusing magnet from a.current source energized by said power supply in series with saidfocusing magnet and with an electron tube having an anode, a cathode, afirst control electrode adjacent said cathode and asecond controlelectrode between said first control electrode and said anodes, meansfor impressing voltage derived from said supply on said second. controlelectrode, and means for so adjusting the voltage impressed on saidcontrol electrodes that the fractional change in current energizing saidfocusing magnet due to changes in said power'source'is substantiallyone-half'the fractional change in" the voltage impressed onsaidaccelerating-a-node.

2. In combination with means for producing an electron beam by means ofan accelerating anode, a focusing magnet forsaid electron beam, a powersupply and means for impressing voltage therefrom on said acceleratinganode, means energized by said supply for impressing voltage on saidfocusing magnet from a current source in;

series relation with said focusing magnet. and with an electron tubehaving an anode, a cathode,

a first control electrode adjacent said cathode and.

a second control electrodebetweenl said first control electrodeandsaidanodes, means forimpressing voltage derived from-'saidsupply'on saidsec-- ond control electrode, and means for so: adjusting:

the voltage impressed on saidi control electrodes that thefractionalchange in current energizing;

said focusing magnet'dueto changesiinasaidpower. source is substantiallyone-half the fractional changein the-voltage impressedion. saidaccelerating anode.

3'. In. combination with means for producingaam electronbeanl by; meansofan acceleratinganode,

a focusing: magnet for' said. electron beam, a. power supply andimeansfor impressing. voltage:

therefrom. on said accelerating anode, means: for" impressingvoltage onsaiict focusing'magnet from; a current: source. energized bysaidtpowersupply:

inseri'es with said focusing magnet and: with. an electron: tube havingan anode,,a cathode, a first control electrode.- ad'jacent said;cathode. and a. second control: electrode between said first con.trolelectrode and: said anodes; means for. impressing; voltage from saidcurrent supply on one ofsaid control. electrodes, and means for'so ad'-justing the voltageimpressed onsaid control elec trodesthatthe-fractional changein current energizing saidl IEOCLlSiBg, magnetdueto changes. in said power source is. substantially one-half thefractional change in. the voltage impressed on said. accelerating,anode.

LYNN.J..ULMAN.

REFERENCES CITED The following references are of record in, the file ofthis patent:

, STATES PATENTS Number Name Date 2;219;l94' Mynall Oct, 22,19402,291,682 Blumlein'et' a1. Aug. 4', 1942

